1. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to a polarization maintaining and/or single polarization optical fiber.
2. Technical Background
Polarization maintaining (PM) and single polarization (SP) optical fibers are useful for ultra-high speed transmission systems and many other applications. One type of prior polarization maintaining fiber includes, as shown in FIG. 1, a central core 10 surrounded by a cladding 11. Core 10 and cladding 11 are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material is greater than that of the cladding material. By way of example only, core 10 may consist of silica containing one or more dopants which increase the refractive index thereof, such as germania. Cladding 11 may comprise pure silica, silica containing a lesser amount of dopant than core 10, or silica containing one or more down dopants, at least one of which is an oxide of an element such as boron or fluorine which lowers the refractive index of silica. In FIG. 1, diametrically opposed relative to core 10, are two regions 12 formed of a glass material (e.g., silica doped with 20 to 25 wt % B) having a coefficient of thermal expansion (CTE) different from that of cladding material 11. When such a fiber is drawn, the longitudinally-extending regions 12 and the cladding regions will shrink different amounts whereby regions 12 will be put into a state of tension or compression depending upon the CTE thereof relative to that of the cladding 11. A strain induced birefringence, which is thus induced by anisotropic thermal stress induced by the mismatch of CTEs between regions 12 and the surrounding regions 11, reduces coupling between the two polarized fundamental modes (with orthogonal polarization directions).
One major drawback of these PM or SP fibers attained through the CTE mismatch (between the cladding 11 and regions 12), is that these fibers are temperature-sensitive due to the fact that the thermal stress changes with temperature. This thermal sensitivity often compromises the stability of the PM or SP performance. Thermal stability of PM an SP fibers is very important for many applications, such as in high-power amps/lasers, high-precision airborne space navigation, and deep-sea (or land) sensor applications, where the fiber can experience dramatic temperature changes during operation. These changes can result in PM/SP characteristic degradation causing deterioration in performance, low navigational accuracy, and possibly a total failure in the device/system if additional, often costly, temperature management is not employed.